This invention relates generally to transdermal drug delivery systems, and more particularly, to pressure-sensitive adhesive compositions for continuous and controlled delivery of active agents over a prolonged period of time that incorporate a cellulose derivative, and in particular esterified or acylated cellulose, to stabilize the concentration of the active agent in the composition and to inhibit crystal formation of the active agent.
The use of transdermal drug delivery systems as a means to topically administer an active agent is well known. Such systems incorporate the active agent into a carrier composition, such as a polymeric and/or pressure-sensitive adhesive composition, from which the active agent is delivered through the skin or mucosa of the user.
In general, transdermal drug delivery systems are either reservoir-type or matrix-type. Both types of systems include a backing layer that forms the protective outer surface of the finished transdermal device and which is exposed to the environment during use, and a release liner or protective layer that forms the inner surface and which covers the adhesive means for affixing the device to the skin or mucosa of a user. The release liner or protective layer is removed prior to application, exposing the adhesive means, which is typically a pressure-sensitive adhesive. The active agent is located between the release liner and backing layer, usually solubilized or dispersed in a solvent or carrier composition.
In a reservoir-type device, the active agent, typically in fluid or gel form, is isolated from the adhesive means used to affix the device to the user. Traditionally, a reservoir system referred to a device having a pocket or xe2x80x9creservoirxe2x80x9d which served to hold the active agent and which was formed in or by the backing layer itself. A peripheral adhesive layer was then used to affix the device to the user. While such devices are still in use today, the term reservoir has become known as a device which employs one or more permeable layers, such as rate controlling membranes and drug permeable adhesives layers, laminated over the reservoir (which is typically nothing more than another layer containing the drug in a carrier composition), in order to more effectively control the delivery rate of the active agent and attachment of the device to the user.
A matrix-type device generally comprises the active agent solubilized or dispersed in an adhesive carrier composition, typically a pressure-sensitive adhesive or bioadhesive, which functions as both the drug carrier and the adhesive means of applying the system to the skin or mucosa. Such devices are described, for example, in U.S. Pat. Nos. 4,994,267, 5,446,070, 5,474,783 and 5,656,286, all of which are assigned to Noven Pharmaceuticals, Inc., Miami, Fla.
A particular advantage over other forms of drug delivery, such as oral administration, is that the transdermal system can provide a continuous and controlled release of the active agent over a prolonged period of time so that the resulting blood levels remain constant.
It has been shown that the degree of saturation and solubility of the active agent in the carrier composition are determining factors in controlling delivery of the active agent from the transdermal system. Since only solubilized active agent is available for delivery out of the transdermal system, the carrier composition must not promote crystal growth or formation, especially during storage of the system prior to use. Generally, active agents have been found to be readily soluble in acrylic polymers. However, in order to deliver a therapeutically effective amount to the system""s user, and to also achieve the desired adhesive strength required for topical application in a matrix-type system, additional polymers and ingredients are often added to the carrier composition (for example, incorporating a rubber, polysiloxane or polyvinylpyrrolidone polymer). Such additional polymers and ingredients can affect the recrystallization of the active agent in the carrier composition. The tendency for crystal formation or growth is known, for example, in the case of steroid hormones.
Formulation of transdermal systems is further frequently hampered by poor solubility of certain active agents in the carrier composition, which in turn also severely limits its therapeutic application. This formulating aspect is particularly difficult in matrix-type systems because the carrier composition has to be optimized not only for the desired active agents but also for the carrier""s pressure-sensitive adhesive properties. While using low concentrations in order to incorporate the active agent into the carrier may not deleteriously affect the carrier""s adhesive properties, low active agent concentration can result in difficulties in achieving an acceptable delivery rate. Poor or inadequate solubility of the active agent further gives rise to crystal formation or growth.
Generally, concentrations of the active agent up to the saturation solubility, and even supersaturated (i.e., an amount of active agent at a concentration greater than the solubility of the active agent in the carrier composition at room temperature) are sought in order to increase or maximize delivery rates. Such systems also allow for continuous administration of the active drug in therapeutically effective amounts for prolonged periods of time, such as greater than 24 hours, and even up to 7 days or more. In such systems, however, the active agent can recrystallize unpredictably, especially during storage. This gives rise to stability problems.
Active agent that is present in crystalline form cannot be delivered through skin or mucosa. Inadequate delivery of the active agent in turn leads to blood levels falling below that which are therapeutically effective. Some transdermal systems rely upon both solubilized and crystalline forms of active agent to achieve the desired drug loading in the carrier composition. Although the drug crystals in such systems are intended to dissolve later, for example after application, such a process is unpredictable and interferes with achieving a controlled delivery rate, especially a zero-order kinetic delivery rate.
Failure to control crystal formation and growth can further interfere with the physical properties of the transdermal system. The presence of crystals, particularly in excessive amounts, can interfere with the carrier composition""s adhesive properties in matrix-type transdermal systems. Furthermore, surface crystals can come into direct contact with the skin or mucosa and promote irritation. The presence of drug crystals is therefore generally undesirable.
It is therefore an object of this invention to provide a transdermal drug delivery system that can substantially suppress or prevent crystallization of active agents incorporated therein for the delivery of a therapeutically effective amount.
It is another object of this invention to provide a transdermal drug delivery system that can substantially suppress or prevent crystallization formation or growth of the active agents incorporated in a pressure-sensitive adhesive carrier composition while retaining good physical adhesive properties.
It is also an object of this invention to provide a transdermal drug delivery system that can incorporate saturated and supersaturated concentrations of the active agent, and deliver the same at a controlled and predictable release rate.
It is a further object of this invention to provide for transdermal drug delivery systems that can incorporate active agents that are insoluble or sparingly soluble in pressure-sensitive adhesives in amounts necessary to deliver a therapeutically effective amount without resulting in recrystallization of the active agent, and deliver the same at a controlled and predictable release rate.
It is still another object of this invention to provide a method for increasing the solubilizing and stabilizing of active agents in transdermal delivery systems.
It is additionally an object of this invention to provide a method for making a transdermal drug delivery system that achieves a substantially zero-order kinetic rate of drug delivery for a prolonged period of time without crystallization of the active agent therein.